The present invention relates to an objective lens, an optical pickup apparatus and an optical information recording and/or reproducing apparatus, and in particular, to an optical pickup apparatus and an optical information recording and/or reproducing apparatus both being capable of recording and/or reproducing information properly for optical information recording media each being different, by using a light source having a different wavelength, and to an objective lens used for the foregoing.
In recent years, there has been researched and developed rapidly a high density disc system capable of conducting recording and/or reproducing of information (hereinafter, “recording and/or reproducing of information” is also represented by “recording/reproducing of information” or “recording and reproducing of information”) by using a violet semiconductor laser having a wavelength of about 400 nm. As an example, in the case of an optical disc conducting recording and/or reproducing of information under specifications of NA 0.85 and light source wavelength 405 nm what is called Blue-ray Disc (hereinafter referred to as BD), information of 23-27 GB per one layer can be recorded for an optical disc with a diameter of 12 cm that is the same as DVD (NA 0.6, light source wavelength 650 nm and storage capacity 4.7 GB) in size, and in the case of an optical disc conducting recording and/or reproducing of information under specifications of NA 0.65 and light source wavelength 405 nm what is called HD DVD (in the other words High Definition DVD and hereinafter referred to as HD), information of 15-20 GB per one layer can be recorded for an optical disc with a diameter of 12 cm. Incidentally, in the case of BD, coma caused by inclination of an optical disc (skew) is increased, thereby, a protective layer is designed to be thinner that that in DVD (thickness is 0.1 mm while that of DVD is 0.6 mm), and an amount of coma caused by skew is reduced. From now on, the optical disc of this kind is called “high density optical disc”.
Incidentally, a value of an optical disc player/recorder is not sufficient when it can conduct only recording and/or reproducing of information properly for the high density optical disc of this type. When it is considered that DVDs wherein various types of information are recorded are available on the market presently, only conducting recording and/or reproducing of information for the high density optical disc is not sufficient, and a product value as an optical disc player/recorder for the high density optical disc is enhanced when recording and/or reproducing of information can be conducted properly also for DVD owned by a user. From such background, an optical pickup apparatus installed in an optical disc player/recorder for a high density optical disc is required to have capability to conduct recording and/or reproducing of information properly while keeping compatibility for both of the high density optical disc and DVD.
As a method to conduct recording and/or reproducing of information properly while keeping compatibility for both of the high density optical disc and DVD, there is considered a method to switch selectively an optical system for a high density optical disc and an optical system for DVD in accordance with recording density of the optical disc for which recording and/or reproducing of information is conducted. However, this method requires a plurality of optical systems, thereby, this method is disadvantageous for downsizing, and cost increase is caused.
For simplifying the structure of the optical pickup apparatus and realizing low cost, therefore, it is preferable to reduce extremely the number of optical parts constituting the optical pickup apparatus by standardizing an optical system for a high density optical disc an optical system for DVD, even in the case of the optical pickup apparatus having compatibility. It is most advantageous for simplification of the structure of the optical pickup apparatus and for low cost to standardize an objective lens arranged to face an optical disc and thereby to make this objective lens to be a single lens. Incidentally, as a common objective lens for plural types of optical discs using light fluxes with different wavelengths each other when recording and/or reproducing of information, there is known an objective lens that has on its surface a diffractive structure having wavelength-dependency of spherical aberration and corrects spherical aberration caused by a difference of a recording/reproducing wavelength and of a protective layer of the optical discs, by utilizing the wavelength-dependency of spherical aberration.
In Patent Document 1, there is disclosed an objective lens of a single lens structure that can conduct recording and/or reproducing of information on a compatible basis for a high density optical disc and DVD.
(Patent Document 1) TOKUKAI No. 2004-79146
The objective lens disclosed by Patent Document 1 is one that has a diffractive structure that generates secondary diffracted light for a violet laser light flux and generates the first order diffracted light for a red laser light flux and corrects spherical aberration caused by a difference of protective layer thickness between a high density optical disc and DVD, by diffracting actions of the diffractive structure. However, this objective lens has two problems mentioned below, although it can be manufactured at low cost because of a single lens structure.
One of the problems is that wavelength-dependency of spherical aberration generated by the diffractive structure is great. In this case, it is impossible to use a laser light source wherein an emission wavelength is deviated from a design wavelength, and a laser light source needs to be selected, which increases manufacturing cost for the optical pickup apparatus. An angle of diffraction of the diffracted light is expressed by                “diffraction order×wavelength/diffraction pitch”.It is necessary to give a difference between angles of diffraction for working wavelengths (where, the working wavelength is also represented by recording/reproducing wavelength hereinafter), for realizing compatibility between optical information recording media each having a different working wavelength by utilizing diffracting actions. “The problem of selection of laser light source” stated above is caused by using the diffractive structure wherein a value of “diffraction order×wavelength” is substantially the same between working wavelengths for a high density optical disc and DVD. In the objective lens disclosed by Patent Document 1, a ratio of “diffraction order×wavelength” between a violet laser light flux and a red laser light flux is 810/655=1.24 to be close to 1 (provided that a unit of wavelength is nm), and therefore, a diffraction pitch needs to be small for obtaining a necessary difference of an angle of diffraction or correcting spherical aberration caused by a difference of protective layer thickness between a high density optical information recording medium and DVD. Accordingly, wavelength-dependency of spherical aberration for the diffractive structure grows greater, resulting in elicitation of “the problem of selection of laser light source” mentioned above.        
Another problem is that vignetting of a light flux on the step portion is caused by formation of a diffractive structure on a highly inclined optical surface, and transmittance is declined by insufficient transfer of a microscopic structure such as a corner section of a ring-shaped zone in the diffractive structure, thus, sufficient efficiency for using light cannot be obtained. Since an inclination of an optical surface grows greater as a numerical aperture of an objective lens grows greater, a decline of the transmittance becomes more remarkable in BD that employs an objective lens having a numerical aperture of 0.85.